BOOST_REGEX_RECURSIVE

Tells Boost.Regex to use a stack-recursive matching algorithm. This is generally the fastest option (although there is very little in it), but can cause stack overflow in extreme cases, on Win32 this can be handled safely, but this is not the case on other platforms.

BOOST_REGEX_NON_RECURSIVE

Tells Boost.Regex to use a non-stack recursive matching algorithm, this can be slightly slower than the alternative, but is always safe no matter how pathological the regular expression. This is the default on non-Win32 platforms.